I was having a discussion with someone about IS and teleconverters. On dSLRs with sensor shift image stabilization, 2X TCs always report the corrected focal length to the camera, so the camera knows the actual focal length and can shift the image sensor an amount appropriate for the longer focal length.

The problem is that few 1.4X TCs report the corrected focal length. This means that the camera may not be shifting the image senor an amount appropriate for the corrected focal length.

Then, it occured to me that optical image stabilization systems in lenses would have no idea that a TC is connected, and would always apply an amount of correction that was too much for the focal length, especially with TC that contains air gaps, and since there are no TCs with only one group, all TCs have air gaps, so all TCs would throw off optical image stabilization systems.

Then it also occured to me that extention tubes are simply one large air gap, and since they never report a change in focal length (because there isn't one), neither optical image stabilization nor sensor shift image stabilization would be able to correctly compensate for camera shake when using extension tubes.

Am I off base here? How would a VR or IS lens know to adjust the amount of compensation for camera shake, given that the focal length is altered by the TC. And how would any image stabilization system know how much compensation to use with extension tubes?

Verrrrrry simple:
The optical stab tries to stab the image from the lens on the focal plane of the lens (not including converters or ext.-tubes) to the max. That in turn is totally independant of the amount of magnification (by converter or extension tube) you're trying to apply "behind" the lens.
So optical stabilization "rulez" ( ) as it is totally immune to everything you mount between the lens and the body

Verrrrrry simple:The optical stab tries to stab the image from the lens on the focal plane of the lens (not including converters or ext.-tubes) ...

In order to do that, it must know how far away the focal plane is. If you increase the distance to the focal plane (with a TC or tube), then the angular deflection imposed by the optical image stabilization will result in too much compensation at the focal plane.

The trick is that the lens does not know and need not know that you changed the focal plane through the use of an extender or converter!
For the lens it's always 43.5mm behind the lens mount (with a Nikon anyway!)
And if it's stable at the "old" focal plane then it's stable at the extended one too

Yep, the lens stabilizes a sharp image and projects that on its "normal" focal plane. The converter just magnifies this (stabilized) image and the extender just makes things close to the lens focus on the sensor.
((I'm not sure that I can express myself correctly and clearly enough ))

You're right, as the sensor has to adapt to the larger magnification of the combo and thus "shift more". So the camera needs to know the magnification pretty precisely to calculate the shift-width.
And if data for this calculation are missing or plain wrong, the body-based IS cannot work effectively.
What I don't know about is whether or not extension-tubes and tele-converters transfer the correct data (distance, focal length)

Extension tubes wouldn't have a corrected focal length to report, because the focal length doesn't change. Most teleconverters report not only the corrected focal length, but the corrected aperture, as can be seen in the EXIF data. So the sensor shift image stabilization has the appropriate info to work with.

But most 1.4X TCs do not report the corrected focal length and aperture to the camera, and so the sensor shift image stabilization system may apply an inappropriate amount of compensation for camera shake.

There is a side benifit to this, however. When using lenses with maximum apertures that permit autofocus, but not with a 1.4X TC, the camera wouldn't know that the maximum aperture is too small for autofocus. It would try to autofocus anyway.

For instance, a 75-300mm f/4.0-5.6 with a 1.4X TC, would effectively be a 112-420mm f5.6-8.0. The f/8.0 maximum aperture at the long end is too small to support autofocus, but the camera would still think the maximum aperture was f/5.6, so it would try. And the exposure would be fine because the meter uses the light it sees, no the light it is supposed to see.

After a little bit of hand-wringing I've decided to move this thread into the Off-topic section. The reason is that as I understand it the DSLR Tips forum is intended to be about the DSLR Tips site, both in terms of feedback and suggestions.

The Off-topic forum is meant to be related in some way to our hobby ( ) so it seemed the best fit, though still not perfect. Maybe we need yet another forum ("Lens talk").